The origin of diversity of antibodies is being analyzed in terms of two parameters: 1. An analysis of the genetics of germ-line variable genes by the use of response and idiotype markers. 2. The selection pressures on germ-line genes by antigen both during germ-line and somatic evolution. The latter entails studies of the mechanism of induction and tolerance as well as the mechanism of the regulation of expression of the class of immune response, cell-mediated or humoral.